1. Field of the Invention
The present invention relates to apparatuses and methods for remotely measuring the temperature of a surface, and has particular utility in the field of laser thermal processing (LTP) where a beam of radiation is scanned across a substrate surface to heat the surface, and has general utility to thermal processing where the temperature of a surface needs to be known to a relatively high degree of accuracy and precision.
2. Description of the Background Art
LTP is a technique for manufacturing semiconductor devices such as integrated circuits or “ICs”. LTP involves irradiating a substrate, such as a doped semiconductor wafer, to bring the substrate surface from a relatively low temperature (e.g., 400° C.) to a relatively high temperature (e.g., 1,300° C.) so quickly that the substrate bulk remains at a low temperature and can pull the surface temperature down quickly. Such a rapid thermal cycle might be used, for example, to efficiently activate dopants in the substrate surface because only the material very close to the top surface of the substrate is heated to the relatively high temperature during irradiation.
As described in U.S. Pat. No. 6,747,245 to Talwar et al., one approach to LTP involves scanning a long, narrow laser beam back and forth across the wafer surface in a raster pattern. The amount of time the laser beam resides over a given point on the wafer surface is called the “dwell time”. Using this scanning approach, it is possible to achieve peak surface temperatures near 1350° C. with dwell times in the millisecond range and below. The result is a rapid thermal annealing of doped Si wafers that yields high activation levels with very little dopant diffusion. When used to fabricate transistor-based circuits, transistors with an abrupt dopant profile and with a low sheet resistance are formed. LTP is capable of providing significantly lower sheet resistance values than possible using so-called Rapid Thermal Processing (RTP), which has dwell times equivalent to several seconds. While the LTP process is quick and effective, the results are sensitive to the peak temperature produced on the substrate surface.
Deviations in peak annealing temperatures may be produced as a result of substrate surface reflectivity variations. Deviations in annealing temperatures may also be an artifact of systemic process deviations associated with the apparatuses used to effect LTP. In either case, a difference in the peak annealing temperature of as little as a 5° C. can result in a measurable and undesirable difference in sheet resistance. Accordingly, it would be advantageous to provide a measurement means to ensure uniform heating during LTP. For example, it would be advantageous to be able to measure the maximum substrate surface temperature during LTP. Because of the rapid heating and cooling cycle associated with LTP, and because of the scanning geometry, in practice the peak surface temperature is best measured remotely. An exemplary remote temperature measuring technology is described in U.S. Patent Application Publication No. 20060255017 to Markle. In addition, it would be advantageous to be able to use the temperature measurement to control the amount of power delivered to the substrate surface.
Nevertheless, opportunities exist for improvements to known technologies relating to remote temperature technologies for LTP applications.